1. Field of the Invention
The invention relates to detecting and quantitating molecules or influences and more particularly to an improved fiber optic photoluminescence sensor.
2. The Prior Art
The use of fiber optics in conjunction with photoluminometry is growing in fields as diverse as biophysics, remote sensing, immunodiagnostics, and chemical process monitoring. Photoluminescence is a well developed, powerful, and versatile technique for chemical or influence (i.e., temperature, pressure, etc.) sensing. Photoluminescence is a broad term which includes fluorescence, phosphorescence, Raman scattering, etc. The intrinsic wavelength difference between excitation and emission makes photoluminescence well suited for use with fiber optics. Fiber optics themselves have unique attributes which make them ideal for many sensing applications. Fiber optics permit remote, continuous monitoring of analytes in hazardous environments, and in the presence of electromagnetic interference or flammable atmospheres. Fiber optics are small and lightweight, making them useful on air-and spacecraft. Fibers have enormous information-carrying capacity due to the THz bandwidth of light, and signals of different colors can travel in the same fiber without interference. The hope (or necessity) of utilizing these advantages has fueled the development of fiber optic sensors employing photoluminometry.
The fundamental idea of photoluminescence-based sensors is to detect an analyte or influence by a change in the photoluminescence of a susceptible material. Several instrument configurations for performing photoluminometric measurements through fiber optics have been described in the literature, using a great variety of photoluminescence observables (intensity, spectra, lifetimes) and configurations for the sensing tip (evanescent wave or distal cuvette). Basically, as shown in the prior art photoluminescence sensor apparatus of FIG. 1, a susceptible photoluminescent material 20 localized at the distal end 9 of an optical fiber is excited by light coming down the optical fiber 8, and its photoluminescence is coupled back into the fiber 8, separated from the excitation, and observed at the proximal end 7 of the fiber 8. In FIG. 1, the solid line/arrows 1 represents the path of the exciting light, while the dashed line/arrows 1' represents the path of photoluminescence.
All fiber optic photoluminescence sensors have a light or excitation source 2, some means for coupling or coupling lens 6 the light into the fiber 8, a photoluminescent material 20 localized at the distal end 9, a means for separating the emission from the excitation 4, and a detector or photodetector 12 (FIG. 1). Fiber optics impose constraints on the optical configuration performing these functions that are not encountered in ordinary photoluminescence sensors, and which require attention to assure optimum performance. For instance, the positions of the excitation source 2, the coupling lens 6 and the proximal fiber end 7 along some axes must be controlled with micrometer precision, which is seldom required in a typical photoluminescence sensor. Also, fiber optic photoluminescence sensors are generally less sensitive than standard research grade photoluminescence sensors, and thus it is important to get the best performance out of the former; this seems to be particularly true for those using a waveguide binding (evanescent wave) sensing tip.
Various types of fiber optic photoluminescence sensors have been proposed (see U.S. Pat. Nos. 4,775,637; 4,582,809; and 4,447,546). Generically, such sensors consist of a light source 2 (FIG. 1), whose exciting light passes through a (spatially or spectrally) filtering mirror 4 and is focused into the fiber 8 by an objective 6 at the proximal end 7 of the fiber 8. The fiber 8 conducts the exciting light to the distal end 9 of the fiber 8, where the photoluminescent material 20 is present or is attached to the end of the fiber 8, where the exciting light is absorbed. The photoluminescent material 20 emits its characteristic emission, which re-enters the fiber 8 (the same fiber need not be used, but typically is) and is conducted back to the proximal end 7 of the fiber 8, where it passes through the objective 6, is reflected off the mirror 4 through a lens 3 and a filter 10 into the photodetector 12. Essentially all the fiber optic photoluminescence sensors described in the literature use this basic scheme, and differ in the details of the components used and their arrangement. For instance, some photoluminescence sensors in the prior art use separate optical fibers to carry the excitation and emission; such sensors have no mirror to separate excitation from emission, but require two objectives, one to direct the excitation into one fiber, and the other objective to receive the emission and focus it onto the detector. Many of the sensors described in the literature are insufficiently sensitive to detect many of the chemical analytes of interest, including pollutants, drugs, and poisons. The improvements described below are aimed at increasing the sensitivity of detecting any analyte or influence, irrespective of the distal end configuration (distal cuvette or waveguide binding), actual sensing chemistry, or wavelengths involved.
Typically, the mirror 4 is a dichroic mirror coated and oriented to pass the exciting light and reflect the (longer wavelength) photoluminescence emission into the detector 12 (or vice versa). Such mirrors have the disadvantages that they are not useful over a broad wavelength range, are a source of background photoluminescence, and have poor transmission. Andrade et al. (U.S. Pat. No. 4,368,047) used a perforated planar mirror to pass the narrow beam of a laser for excitation, and reflect the more broadly spread photoluminescence as it comes back out of the objective 6, towards the detector 12. Braun (U.S. Pat. No. 4,533,246) also discloses the use of a perforated planar mirror. The disadvantage of this is that it requires a separate lens 3 to focus the photoluminescence on the detector 12, which adds weight, complexity, insensitivity, and a propensity for misalignment to the sensor.
The purpose of the filter 10 in FIG. 1 is to block scattered shorter wavelength exciting light from entering the detector 12 and being confused with authentic (signal) photoluminescence. Such light scattered off the coupler or other components can be orders of magnitude stronger than the actual photoluminescence, and can seriously degrade the performance of the sensor. The colored glass or interference filters well known to the art will ordinarily serve in this respect. Unfortunately, nearly all of these filters themselves photoluminesce appreciably when struck by scattered exciting light, and this photoluminescence can be sensed as authentic sample photoluminescence.
The use of a chopper 16 or other light modulator together with a lock-in amplifier 14 or other phase-sensitive detector is well known in the art for improving the detectability of weak signals, such as in fiber optic sensors. Thus, a chopper 16 placed in the beam of exciting light will modulate it at a particular frequency, and the lock-in amplifier 14 can be tuned to measure the detector 12 output at only that frequency, eliminating spurious noise at other frequencies. Ordinarily, the chopper 16 is placed as closely to the light source 2 as is convenient.
Many sorts of lenses or objectives have been used to launch light into fiber optics, including gradient index rod lenses, simple lenses, spherical lenses, and most often, refracting microscope objectives. All of these optics are transmissive, and therefore suffer from two drawbacks: most transmit ultraviolet light poorly, and due to their transmissive nature they can photoluminesce when light passes through them. Ultraviolet excitation is very useful for detecting many photoluminescent molecules.
Many kinds of light detectors have been used to detect the photoluminescence signals. They include photomultipler tubes, PIN photodiodes, avalanche photodiodes, and phototransistors. Their usefulness is mainly determined by their sensitivity, which is well known in the art.
The advantages of fiber optical photoluminescence sensors per se are well known: they permit continuous monitoring of a variety of chemical analytes under circumstances inhospitable to conventional analytical chemical techniques or instrumentation. For instance, fiber optic photoluminescence sensors have been designed to sense carbon dioxide or pH in the bloodstream, pollutants deep underground, or toxic chemicals in the air. All of them have the same functional requirements as outlined in FIG. 1, although they differ in detail, and selection of components.